Don't use swiftcast, just hardcast convert if you need to. If you get a proc, use it earlier so you can weave swift and convert. If you hardcast swiftcast on a Fire IV, you don't actually gain any potency because of the time to cast Swiftcast takes up the .5s you gain from a swiftcasted Fire IV. Either way, it's not worth the extra .5 seconds to use Swiftcast, save it for movement later, cause you'll have movement to do I guarantee.